The present application claims priority under 35 U.S.C xc2xa7 119 based upon Swiss Patent Application No. 2002 0411/02 filed on Mar. 8, 2002.
The invention concerns a method and an apparatus for dispensing solder on a substrate.
Soldering methods of this type are used typicallyxe2x80x94however not exclusivelyxe2x80x94for the mounting of semiconductor chips onto a metallic substrate, a so-called leadframe. As a rule, power semiconductors are mainly connected to the substrate, which normally comprises copper, by means of soft soldering in order to guarantee a more effective dissipation of the heat loss created during production via the solder connections as compared to mounting by means of adhesive. However, above all with increased power density, high demands are placed on the homogeneity of the solder connections, ie, defined density, uniform spread and perfect wetting of the solder layer over the entire chip surface and complete freedom from bubbles as well as purity of the soldered connection are demanded; on the other hand, the solder must not exude laterally out of the soldered joint and spread out next to the chip which again necessitates the exact dosing and positioning of the solder portion.
In the application area of chip assembly, a method is widely used in practice with which the end of the soldering wire is brought into contact with the substrate which is heated to above the melting temperature of the solder in order to melt off a piece of the wire. Because of its simplicity and flexibility, this method is well suited for mass production. However, the melting process, above all the volume and the positioning of the solder portion, is very dependent on the material combination and the surface characteristics of the substrate, ie, the amount and position of the respective solder to be melted are subject to considerable variations. Furthermore, the roughly circular shaped wetted area created is badly adapted to the rectangular or square form of the chips. In addition, a forming stamp is known from U.S. Pat. No. 6,056,184 with which the solder portion deposited on the substrate can be brought into a flat, rectangular form adapted to the shape of the chips.
A method with which liquid solder is injected into a cavity formed between the forming stamp and the substrate is known from U.S. Pat. No. 5,878,939.
These known methods have several disadvantages. The shape of the deposited solder is either round or a specific forming stamp must be manufactured for each rectangular shape. Such a forming stamp has side walls which occupy a part of the substrate. Therefore, the solder can not be applied up to the edge of the chip island which is to accept the chip. Furthermore, the substrate must be heated to above the melting temperature of the solder and the deposited solder must be kept in liquid form from its application up to depositing the semiconductor chip. It is also of disadvantage that the parts which come into contact with the liquid solder have to be regularly cleaned for which purpose production has to be interrupted.
A method with which flat formed bodies made of soldering metal, so-called xe2x80x9csolder preformsxe2x80x9d, are prefabricated, the dimensions of which are adapted to the chips is known from U.S. Pat. No. 4,577,398 and U.S. Pat. No. 4,709,849. The solder preforms are then placed onto the substrate and melted by it in order to form a soldering layer of the required dimensions. However, because of the necessary prefabrication of the solder preforms and the additional assembly operations, this method is very costly and not very flexible.
The object of the invention is to develop a method for applying solder to a substrate which no longer demonstrates the named disadvantages.
The method in accordance with the invention for dispensing solder on a substrate distinguishes itself in that a soldering wire and a forming gas heated to a predefined temperature are supplied to a mixing chamber of a two-component nozzle so that, in the mixing chamber, solder is melted off and carried with the gas stream. The solder blown out of the two-component nozzle settles on the substrate. The two-component nozzle is moved or swivelled relative to the substrate in order to deposit the blown out solder onto the predetermined area of the chip island. The soldering wire is preferably fed to the mixing chamber in a cooled guide pipe. To interrupt the melting of the solder, it is preferred to pull back the soldering wire so that the end of the soldering wire is located within the guide pipe.
The invention enables the contactless application of solder onto a substrate so that no scratches can occur on the substrate. In addition, the pattern of the deposited solder can be freely programmed and solder can be applied up to the edge of the chip island. The solder can be deposited without the substrate having to be heated up to the melting temperature of the solder. The distribution of the deposited solder on the substrate can therefore be controlled by a camera without heat-related waviness blurring or distorting the picture.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention. The figures are not to scale.